Interleukin-12 (IL-12), formerly known as cytotoxic lymphocyte maturation factor or natural killer cell stimulatory factor, is a 75-KDa heterodimeric cytokine composed of disulfide-bonded 40-KDa (p40) and 35-KDa (p35) subunits that has multiple biological activities including stimulation of the proliferation of activated T and NK cells (Gately, M. K., et al., 1991, J. Immunol., 147:874) (Kobayashi, M., et al., 1989, J. Exp. Med., 170:827), enhancement of the lytic activity of NK/LAK cells (Kobayashi, M., et al., 1989, J. Exp. Med., 170:827; Stern, A. S., et al., 1990, Proc. Natl. Acad. Sci. USA, 87:6808), enhancement of cytolytic T-cell responses (Gately, M. K., et al., 1992, Cell. Immunology, 143:127), induction of interferon gamma by resting and activated T- and NK-cells (Kobayashi, M. et al., 1989, J. Exp. Med., 170:827; Chan, S. H., et al., 1991, J. Exp. Med., 173:869), and promotion of T.sub.h 1-type helper cell responses (Manetti, R., et al., 1993, J. Exp. Med., 177:1199; Hsieh, C.-S., et al., 1993, Science 260:547).
The biological activity of IL-12 is mediated by the binding of the IL-12 molecules to cell surface, or plasma membrane, receptors on activated T- and NK cells; however, the contributions of the individual subunits, p35 and p40, to receptor binding and signal transduction remain unknown. Studies with labeled IL-12 have shown that this binding occurs in a specific and saturable manner. IL-12 delivers a signal to target cells through a receptor that was initially characterized on phytohaemagglutinin (PHA)-activated CD4+ and CD8+ T-cells and on IL-2 activated CD56+ NK-cells (Chizzonite, R., et al., 1992, J. Immunol., 148:3117; Desai, B., et al., 1992, J. Immunol., 148:3125).
A survey of over 20 human cell lines belonging to the T-, B-, NK- and myelomonocytic lineages only identified a single CD4+, IL-2 dependent human T-cell line (Kit 225/K6) that constitutively expresses the IL-12 receptor and responds to IL-12 (Desai, B., et al., 1992, J. Immunol., 148:3125; Desai, B., et al., 1993, J. Immunol. 150:207A). Freshly prepared PHA-activated peripheral blood mononuclear cells (PBMC) and the Kit 225/K6 cell line thus represent two convenient cell sources to study the biochemistry of the functional IL-12 receptor; there may be others.
Equilibrium binding experiments with .sup.125 I-labeled IL-12 identified three sites with binding affinities for human IL-12 of 5-20 pM, 50-200 pM, and 2-6 nM on IL-12 responsive T-cells (Chizzonite, R., et al., 1994, Cytokine 6(5):A82a).
A cDNA encoding a low affinity IL-12 receptor was previously cloned (Chua, A., et al, 1994, J. Immunology 153:128; U.S. patent application Ser. No. 08/248,532, filed May 31, 1994 (incorporated herein by reference)). Based on a previously suggested nomenclature (Stahl and Yancopoulos, 1993, Cell 74:587), we now call the initially isolated human IL-12 receptor chain the beta1 chain. However, because (i) this isolated human IL-12 beta1 receptor chain binds human IL-12 with low affinity, and (ii) IL-12 responsive T-cells have a high affinity binding site for human IL-12, another human IL-12 receptor chain must exist.